In order to follow Sébastien's advice, the `AllgoUser` model take a
default value on the token rather than overriding the save method in
order to prevent any issues if a user is generated by bypassing the
django user model.

I changed the view in order to generate a new token each time the user
wants to do so using a HTML form. This change follows the token
generation modification in the model.

Because we don't use the rest_framework anymore, I removed the call
inside that form.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>